This invention relates to an optical head and an associated laser emission unit for reading from and writing into an optical memory medium such as an optical disk or the like, and more particularly to an optical head having a smaller size which is preferably used in an optical memory device of the type having a reduced size and light weight.
In recent years, electric and electronic equipment and devices have been reduced in size. Optical memory devices with an optical memory medium such as an optical disk etc., have also been reduced in size. In order to realize a smaller device, it is important to reduce the size of an optical head because installation and moving space for the optical head can be reduced if smaller optical head is used. Further, if the weight of the small optical head becomes light, transporting mechanism for the optical head can be reduced, and very small memory device can be provided. In addition, if smaller and lighter optical head is used, positioning of the optical head in the memory device can be finely controlled for reading and writing, and writing and reading error can be minimized.
FIG. 46 shows a conventional optical head. In the optical head, a housing 7 contains a laser emission unit 20 with a laser diode. A prism 92 perpendicularly changes the direction of the laser beam emitted from laser emission unit 20 and passing through a collimater lens 94. A laser output power detection element 15 detects a part of the laser beam passing through prism 92 to determine the output power of laser emission unit 20. A diffraction element 11 diffracts a laser beam (returning beam) reflected from an optical disk provided outside of the housing. A data detection element 14 detects a reflected beam diffracted by diffraction element 11. A mirror 61 changes the direction of laser beam angled by prism 92 to an objective lens 12 through which the laser beam is sent out. The returning beam from the optical disk returns into the optical head again via objective lens 12. The returning beam passes through mirror 61, prism 92 and diffraction element 11, and converges on data detection element 14 for detecting a position error signal and recorded signal. Objective lens 12 is supported by a position control mechanism 93 in order to achieve a fine focusing control of the laser beam. Control mechanism 93 is mounted on supporting wall 4 of housing 7 by supporting springs 3. Control mechanism 93 is provided with focusing and tracking coils (not shown). Control mechanism 93 is also provided at both sides with two magnet plates which are located opposite to these coils. Therefore, the position of the position control mechanism can be precisely controlled by applying a current to the coils.
In order to reduce the coherence of the laser beam and to suppress laser noise, it is a usual practice to superpose a high frequency wave component on a driving electrical current supplied to the laser diode. For this purpose, the laser noise reduction element 5 is arranged beside laser emission unit 20. Since noise is likely to leak out from laser noise reduction element 5, laser noise reduction element 5 is enclosed by a shielding case 95 to prevent the noise leakage.
It may be possible to eliminate some of the above elements from the optical head, or to use another method with reduced number of elements having functions equivalent to the functions of some of the above elements, in order to reduce the size of the optical head. In FIG. 47, another laser emission unit is shown. It has a different type of laser power detection element which detects backwardly emitted laser beam for detecting the laser power. This laser emission unit is a canned type unit having a disk-shaped stem 24, a radiator 22 projected from stem 24, a semiconductor laser (laser diode) 21 mounted on the top of radiator 22, a cylindrical-shaped cap 25 covered on stem 24 for protecting laser diode 21 and radiator 22, and a circular emitting window 26 positioned onto the central portion of cap 25. The laser diode is positioned just below the center of window 26 through which the laser beam is emitted. In addition, a laser power detection circuit 23 is disposed on stem 24 for detecting the backwardly emitted laser beam.
When this laser emission unit is used, since the optical head is not provided with a laser power detection element, it can be reduced in size by that amount. However, the power of backward beam is not usually proportional to that of forward beam. Therefore, in order to get the accurate power level, it is necessary to install a power detection element as in the conventional optical head shown in FIG. 46. Hence, up to now, there has not yet been developed an optical head having smaller size while maintaining high performance such as resolution, etc.
As mentioned above, it is important to have smaller optical head in order to provide still smaller optical memory device. However, with the data density becoming higher, performance of optical head must not be degraded.